The present invention relates generally to optical devices. More specifically, the present invention relates to a mode expander with a co-directional grating.
Telecommunications is a large and fast growing industry. This growth has been enabled, at least in part, by the use of optical amplifiers to extend the range of long-haul optical transmission and by the deployment of Dense Wavelength Division Multiplexing (WDM) systems to increase the optical channel capacity. These technologies have been improved by extending the bandwidth of Erbium-Doped Fiber Amplifier (EDFA) optical amplifiers from about 20 nm to about 80 nm.
A technical challenge in extending the bandwidth of EDFAs has been the need to provide enough gain in the EDFAs over the entire bandwidth. This can be accomplished by single-mode 980 nm or 1480 nm laser pumps with output powers on the order of 1 Watt or more. Known laser pumps for EDFAs are single-mode semiconductor narrow-stripe pump lasers limited to about 150 mW in a single-mode fiber. This limitation is due to catastrophic optical damage that can occur if the optical power density at the facet of the laser exceeds about 1 to 2xc3x97107 W/cm2 and due to the fact that stripe laser is narrow (e.g., 3-5 xcexcm) to ensure single-mode operation.
To overcome the limitation relating to the optical power density, the cross-section area of the laser""s active region must be widened to reduce the power density in the laser stripe (i.e., the active region), especially at the facet of the laser. Widening the cross-section area of the laser""s active region, however, conflicts with the design constraint of single-mode operation. Thus, a need exists to increase the cross-sectional area of the portion of an output facet of a laser through which light is transmitted while maintaining single-mode operation. More generally, a need exists to couple light from one waveguide having, for example, a narrow oval-like cross-sectional area, to another waveguide having, for example, a larger cross-sectional area.
An apparatus comprises a first waveguide, a second waveguide and a transitional coupler. The first waveguide is defined by a first material and a second material. The first waveguide has a core and a cladding. The first material is the core of the first waveguide, and the second material is the cladding of the first waveguide. The second waveguide is defined by a third material and a fourth material. The second waveguide has a core and a cladding. The third material is the core of the second waveguide, and the fourth material is the cladding of the second waveguide. A cross-section of the core of the second waveguide is larger than a cross-section of the core of the first waveguide. The transitional coupler has a first end integrally formed with and operationally coupled to the first waveguide, and has a second end integrally formed with and operationally coupled to the second waveguide. The transitional coupler includes a co-directional grating.